The present invention relates to carrier particles of a two-component dry-type developer for developing latent electrostatic images to visible images for use in electrophotography, electrostatic recording methods and electrostatic printing methods. More particularly, it relates to carrier particles coated with a silicone resin layer comprising a silicone resin, an organic tin compound and finely-divided electroconductive particles.
Conventionally, as the methods for developing latent electrostatic images with toner, for example, a cascade development method (U.S. Pat. No. 2,618,552) and a magnetic brush development method (U.S. Pat. No. 2,874,063) are known. In either method, a two-component developer is employed, which comprises carrier particles and toner particles, and the toner particles are usually much smaller than the carrier particles and are triboelectrically attracted to the carrier particles and are held on the surface thereof. The electric attraction between the toner particles and the carrier particles is caused by the friction between the toner particles and the carrier particles. When the toner particles held on the carrier particles are brought near or into contact with a latent electrostatic image, the electric field of the latent electrostatic image works on the toner particles to separate the toner particles from the carrier particles, overcoming the bonding force between the toner particles and the carrier particles. As a result, the toner particles are attracted towards the latent electrostatic image, so that the latent electrostatic image is developed to a visible toner image. In this case, it is necessary that the toner particles be charged with an appropriate polarity and with an exact amount of electric charge, so that the toner particles are preferentially and exactly attracted to the desired areas to be developed on the photoconductor.
In a conventional two-component type developer, it is apt to occur that the surface of the carrier particles is eventually covered with the toner particles to form a toner film layer on the carrier particles in the course of the mechanical mixing of the toner particles with the carrier particles in the development apparatus. Once this phenomenon takes place, which is generally referred to as the "spent phenomenon", the toner particles gradually accumulate on the carrier particles, so that the triboelectric charging between the carrier particles and the toner particles is replaced by the triboelectric charging among the toner particles. In the end, the triboelectric charging characteristics of the developer significantly deteriorate, so that a considerable amount of the toner particles are deposited on the background of the copy images. As a matter of course, when this occurs, the copy quality is considerably degraded. In the end, it is necessary to replace the entire developer by a new developer.
In order to prevent the spent phenomenon, a method of coating the surface of carrier particles with a variety of resins has been proposed. However, resins capable of satisfactorily preventing the spent phenomenon have not been discovered. At one extreme, for instance, carrier particles coated with a styrene - methacrylate copolymer or polystyrene are excellent in the triboelectric charging properties. However, the surface energy of such carrier particles is comparatively so high that the carrier particles are easily covered with the toner particles while in use. In other words, the spent phenomenon occurs easily and, accordingly, the life of such developer is short.
The above-mentioned "spent phenomenon", is greatly reduced with carriers coated with a polytetrafluoroethylene polymer, since such carriers have a low surface energy. However, since the polytetrafluoroethylene polymer is positioned on the extreme negative side in the triboelectric series, the carriers coated with the polytetrafluoroethylene polymer cannot be employed when toner particles must be charged to a negative polarity.
In order to eliminate the above shortcomings, it has been proposed to coat the carrier particles with a material having a low surface energy, for example, with a silicone resin as proposed in Japanese Patent Publication No. 44-27879 and Japanese Laid-Open Patent Application No. 50-2543. In this method, the deposition of the toner particles on the carrier particles can be prevented. However, the silicone resin easily wears away and lacks mechanical strength. Therefore, when the carrier particles coated with silicone resin are used for continuous copying over an extended period of time, the core materials of the carrier particles are exposed since the silicone resin layer wears away by the collision among the carrier particles themselves and between the carrier particles and the mechanical parts of the development apparatus. As a result, the triboelectric charging between the toner particles and the silicone resin is eventually replaced by the triboelectric charging between the toner particles and the core materials of the carrier particles. Once this takes place, the triboelectric characteristics of the developer cannot be maintained constantly. Accordingly the copy image quality is significantly degraded. Further, since most of the resins for use in coating the carrier particles have high electric resistivity, when carrier particles coated with such resins are used in the developer, problems such as edge development, a significant decrease in image density and no image formation could occur.
Such coated carrier particles can be improved by decreasing the electric resistance of the coated layer of the carrier particles, for instance, by dispersing an electroconductive material in the coated layer of the carrier particles.
More specifically, when the carrier particles are provided with a certain electroconductivity, the carrier particles work as a development electrode, so that development of latent electrostatic images can be carried out as if development electrodes were positioned in close contact with the electrophotographic photoconductor. The result is that not only line images, but also solid images can be reproduced faithfully to the original images.
Conventionally, as such electroconductive materials for use in the coated layer of the carrier particles, for example, carbon and tin oxide are employed. However, when such electroconductive materials are employed in the coated layer of the carrier particles, the electric resistivity of the carrier particles is so decreased that the electric charge generated in the toner particles leaks through the carrier particles which are in contact with the toner particles, so that the toner particles cannot maintain a predetermined necessary amount of electric charge for development.
In order to develop latent electrostatic images formed on a photoconductor with a toner, the toner must maintain a predetermined quantity of electric charge. It is generally said that the quantity of electric charge ranging from 10 .mu.C/g to 20 .mu.C/g is suitable, since when the electric charge quantity is less than 10 .mu.C/g, fogging appears in the developed images or the developer is caused to scatter from the development apparatus. On the other hand, when the electric charge quantity is more than 20 .mu.C/g, an image density which is sufficiently high for faithful reproduction of original images cannot be obtained.
When the electric resistivity of the carrier particles is decreased, the quantity of electric charge that can be generated in the toner particles also decreases. When carrier particles with an electroconductive material dispersed therein are employed, it is necessary to appropriately adjust the quantity of electric charge to be generated in the toner. In order to do this, a charge controlling agent, for instance, a dye conventionally employed as charge control agent, is added to the toner, by dissolving such a dye in a solvent together with a resin component of the toner, or by kneading the dye together with a resin component of the toner.
Dyes employed as such charge controlling agent are generally expensive, and when a small amount of the dye is employed, it does not work effectively as charge controlling agent, while when a large amount of the dye is employed in order to increase the electric charge quantity, it becomes extremely difficult to uniformly disperse the dye throughout the resin and when such a toner is employed for an extended period of time, the development characteristics are significantly degraded while in use and high image quality cannot be obtained in a stable manner.